Hollow fiber devices for fluid separations are well known in the art. Such devices are used extensively in gas separation applications including hydrogen separation from refinery gas streams, acid gas separations from natural gas and nitrogen generation from air, and numerous liquid operations such as reverse osmosis desalination or dialyses processes to name a few. Hollow fiber membrane chemistry, morphology, module design and construction methods are optimized with respect to specific membrane separation application.
Improved flow distribution is frequently a major concern in design and construction of hollow fiber membrane devices. Uniform flow distribution without channeling or bypassing is frequently accomplished by controlled and uniform distribution of hollow fiber packing density. Examples of such hollow fiber construction methods can be found in U.S. Pat. Nos. 3,690,465, 3,755,034, 4,631,128, 4,881,955, 4,865,736 and 5,284,584 for example.
One particularly advantageous method of constructing hollow fiber devices with controlled and/or uniform distribution of packing density is by winding methods. A description of such methods can be found in, for example, U.S. Pat. No. 3,794,468, 4,207,192, 4,336,138, 4,631,128 and 4,881,955. U.S. Pat. No. 4,336,138 and 4,430,219 teach that one may wind a separate thread or tape into the module in the vicinity of the ends of the module in order to fix the hollow fibers to the core at the terminal portions. These references make no mention of using or extending the thread into other portions of the module.
In some separation applications it is particularly advantageous to construct wound hollow fiber devices with fibers wound at high wind angles, .theta.. The term "wind angle" is defined as follows: With a module in the horizontal position, the wind angle .theta. is defined as the angle at which the fiber is laid across the module with respect to the vertical axis. For example, fibers wound at a 90.degree. wind angle would be parallel to the longitudinal axis of the module and straight from end to end on the module. Fibers wound in a module at the same wind angle will have the same fiber length. A high wind angle is one in which .theta. is greater than 45.degree.. When bundles are constructed by winding hollow fibers at high winding angles as compared to low winding angles, the bore-side pressure drop is decreased in bundles of equal length. However, hollow fiber bundles wound at high wind angles can become structurally unstable when operated under high fluid linear velocity conditions on the shell side of the hollow fiber membrane, particularly when the treated fluids are viscous. Redistribution and shifting of hollow fibers can occur leading to flow channeling and bypassing.
Though wound hollow fiber devices exhibit improved and more uniform flow distribution characteristics, some flow maldistribution, such as entrance and exit flow nonuniformities, can still occur. Thus there still remains a need to increase structural stability and to further improve flow distribution in wound hollow fiber membrane devices.